1. Field of the Invention
The present invention relates to a variable optical attenuator used for attenuating light in optical communications.
2. Background of the Invention
Up to now, optical attenuators used in an optical communication system or an optical measuring system are mainly structured as the following three types. A first type is structured such that optical fibers are so arranged as to face each other at an interval, and a magneto-optical crystal is disposed between those optical fibers. A second type is structured to utilize a thermo-optical effect. A third type is structured such that optical fibers are disposed to face each other at an interval, and a medium having an optical attenuation function such as an optical attenuation film or an optical attenuation plate is disposed between those optical fibers.
The optical attenuator structured to provide the above optical attenuation film is, for example, shown in FIG. 10. A tabular glass substrate 1 has a light input face 3 and a light output face 4 which are in parallel with each other. The surfaces of the glass substrate 1 (light input face 3 and light output face 4) are formed, for example, with antireflection films 14a and 14b which are about 0.1% in optical reflectivity. The light input face 3 is formed with an optical attenuation film 2.
The optical attenuation film 2 shown in the figure has a film thickness distribution in an x-direction of the figure. The film thickness distribution has an optical attenuation factor distribution (a change in the optical attenuation factor) in the x-direction. The optical attenuation film 2 may be formed with the optical attenuation film 2 on at least one face of the light input face 3 and the light output face 4.
However, in the above-mentioned conventional optical attenuator, the light input face 3 and the light output face 4 of the glass substrate 1 are in parallel with each other. For that reason, the light interferes with each other between the light input face 3 and the light output face 4 in a multiplexing manner. As a result, as shown in FIG. 11, the optical attenuation factor of the optical attenuator has the wavelength dependency.
That is, the optical attenuation factor of the optical attenuator periodically changes depending on the wavelength, and a difference between the maximum value and the minimum value of the optical attenuation factor becomes about 0.3 dB. In the light transmission spectrum shown in the figure, the period of the wavelength dependency of the optical attenuation factor is set to 1.5 nm.
Japanese Patent No. 2933919 discloses a structure in which the light input face 3 and the light output face 4 of the glass substrate 1 are not in parallel but tapered. The structure has a glass substrate 1 and an optical attenuation film 2 as in the device shown in FIG. 10.
According to the present invention, there is provided an optical attenuator comprising;
a substrate having a light input face and a light output face with an angle defined by the light input face and the light output face of said substrate being set to be within a scope from 0.15xc2x0 to 0.5xc2x0; and
an optical attenuation film attenuating an input light with a predetermined optical attenuation factor, formed on at least one face of said light input face and said light output face of said substrate.
According to the present invention, there is provided an optical attenuator module, comprising:
an optical attenuator as set forth in any one of claims 1 to 9;
a first optical fiber that receives a light which is to be inputted to said optical attenuator from an external;
a second optical fiber that inputs the light outputted from said first optical fiber and passing through said attenuator, and outputs the light to the external; and
an optical attenuator moving means that moves said optical attenuator.